1. Technical Field
The present invention relates to a display apparatus with a backlight, such as a liquid crystal display (LCD), and in particular, to both a display apparatus that uses a gas discharge tube as the backlight and a method of driving the gas discharge tube.
2. Related Art
A conventional LCD is usually provided with a backlight unit and a liquid crystal shutter placed on the backlight unit. The backlight unit is composed of various constituents including a straight tubular type or an L-shaped type of cold cathode discharge lamp, a light-guide plate, a reflection plate, and a polarization plate, thus emitting light of a certain luminance. The liquid crystal shutter is subjected to electrical control for adjusting its light transmittance. The light emitted from the backlight unit is used to display images of arbitrary gradations in response to light transmittances of the liquid crystal shutter.
However, in cases where such LCDs are used in monitor devices of televisions or personal computers, moving pictures are disturbed due to the liquid crystal shutter, because the shutter has a response speed as slow as the order of milliseconds. For displaying a moving picture, picture disturbances result in that image-pixel information which should be displayed during the last field remains at some pixels of an image to be displayed during the current field, even though those pixels should be excluded from luminescence during the current field. In such situations, image information is displayed erroneously with blurs and/or runs, which degrades image quality largely.
FIGS. 1A and 1B explain such disturbances of images, in which the axis of ordinates shows the time, thus one grid expressing a single field, while the axis of abscissae shows both pixel positions in an LCD image and luminances at each pixel. The graphs in FIGS. 1A and 1B depict images viewed by the human eyes under the condition that images moved two pixies per field.
FIG. 1A explains a situation where the response speed of the liquid crystal shutter is 0 msec, which is the ideal response speed. In this ideal case, there are no temporal changes in the luminances at each pixel during each field. Hence images whose temporal and spatial luminances are uniform are moved to specified position field by field. Accordingly, the images on an LCD monitor can be viewed by the human eyes as they are, without blurs resulting from the response speed. An exception is that, differently from CRT monitors, the LCD monitors provide images blurred, to some extent, by the fact that the luminescence lasts during the entire period of each period. This is because, when moving pictures are displayed, the lasting luminescence during each field, without a rest, causes images to be elongated in the moving direction of the images, and such elongated images are subjected to observation of the human eyes.
Meanwhile, FIG. 1B explains another situation where the rise time of the liquid crystal shutter is 16 msec. Moving pictures to be displayed are able to reach their desired luminances after a period of time of 16 msec due to the response time. Then, after another time of 16 msec., the liquid crystal shutter has a light transmittance of 0%. This results in that an image which should be displayed only during one field is forcibly visualized over the two fields. Thus, if a moving picture that moves in the frame every field is displayed, the human eyes observe images which are enlarged in the moving direction of the images, that is, blurred in the that direction. Quality of images displayed on an LCD monitor is therefore deteriorated badly.
In addition, a conventional flat type of backlight for LCD units can be driven according to a technique shown in FIG. 2, in which two types of sustaining voltage pulses whose cycles are different by half a cycle from each other are applied to sustaining electrodes during a luminescence period. The sustaining voltages are responsible for sustaining discharges. Additionally, as shown in FIG. 2, to make the discharge stable in the next lighting period, voltage pulses for pilot discharges, which have amplitudes less than those of the sustaining voltage pulses, are applied to the sustaining electrodes during a luminescence-rest period. Applying such pilot-discharging voltage pulses in the luminescence-rest period also generates a certain level of luminance, not a luminance of zero, on the screen during the luminescence-rest period.
When such an LCD is mounted on televisions or personal computers as their monitors, a poor response of the LCD will cause a problem, due to the fact that there is a certain level of luminescence during the luminescence-rest period. The response of the shutter incorporated in the LCD is generally limited to the range from a few milliseconds to several tens of milliseconds. Hence, to display moving pictures whose contents change every field (i.e., every period of 16.7 msec) is liable to being mixed with the image of the last field, thus blurs or runs still being present in images.
Further, when elongating the repetition time of both the lighting period and the luminescence-rest period to as large as 16.7 msec, the lighting is made unstable.